This invention relates to the production of ferroelectric memory, and, more particularly, to the production of ferroelectric memory incorporating oriented PbZr1xe2x88x92xTixO3 thin films.
Embedded memory applications bring together two different silicon technologies, logic and memory, presenting new challenges for device integration. To date, there have been many publications and patents on discrete ferroelectric (FE) capacitors for use in memory devices. However, commonly used FE materials, such as lead zirconate titanate (PZT), are piezoelectric, that is, their electrical properties vary in response to mechanical stress or physical distortion. In addition, they exhibit a physical distortion when an electric field is applied. This distortion may alter the charge storage properties of the material. Thus, as ferroelectric materials are embedded into devices containing four to five layers comprising various materials, the practitioner must be concerned with the electrical effects of stresses imposed on the ferroelectric layers. Memory applications require robust dielectric materials that are insensitive to fluctuations in stress resulting from deposition of subsequent layers. Dielectrics, including PZT and other ferroelectric materials, that are used in semiconductor memory must exhibit electrical properties independent of imposed external stresses. That is, the interaction between mechanical stress (or volume) and voltage must be reduced.
The majority of researchers studying ferroelectric thin films for memory applications employ tetragonal PZT materials as the storage medium because the remnant polarization is larger in the tetragonal phase than in other phases of PZT. In addition, these films are easier to produce than films incorporating other phases of PZT. However, tetragonal films require high drive voltages because of their relatively high coercive fields. In contrast, the current trend is to reduce the operating voltage of devices. The drive voltage can be reduced by decreasing film thickness, but these thin films are frequently not able to store charge reliably. Dielectric materials are required that can be utilized at lower voltages (i.e., lower coercivity materials) and that will exhibit electrical properties independent of imposed external stresses.
In one aspect, this invention is a multi-layer electrical device, for example, a capacitor or a transistor, including a dielectric layer and an electrically conductive layer in electrical communication with one another. The dielectric layer comprises a piezoelectric material for which the composition and orientation are chosen to minimize the effect of mechanical stresses imposed by the other layers of the device on the electrical properties of the piezoelectric material in the dielectric layer. In addition, the composition of the layer is optimized to maximize the number of available domains that are oriented along a projection of a polarization dipole of the piezoelectric material, increasing the number of domains in the layer that are available for charge storage. In a preferred embodiment, the piezoelectric material is a ferroelectric material having the composition PbZr1xe2x88x92xTixO3 (PZT), where x is between 0.15 and 0.4. As a result, the ferroelectric material has a rhombohedral unit cell. The PZT may be deposited with a (111) orientation. The device may also include an underlying layer on which the PbZr1xe2x88x92xTixO3 is deposited. The layer has an interatomic spacing, i.e., between 0.37 and 0.45 nm, compatible with that of the interatomic spacing within a plane with respect to which the deposited PbZr1xe2x88x92xTixO3 is to be oriented. The layer may comprise platinum or iridium.
This invention is also directed to an oriented thin film comprising a dielectric material having an orientation wherein characteristics of the film such as composition and orientation are optimized to minimize the interaction between voltage across and mechanical stress on the dielectric material. In a preferred embodiment, the dielectric material comprises a ferroelectric material. Again, the material may have the composition PbZr1xe2x88x92xTixO3, where x is between 0.15 and 0.4, resulting in a rhombohedral unit cell. The PZT may be deposited with a (111) orientation.
In another aspect, the invention is directed to an electrical device incorporating a dielectric layer that includes a piezoelectric material. The piezoelectric material comprises a ferroelectric material of the composition PbZr1xe2x88x92xTixO3. Such a PZT material, where x is between 0.15 and 0.4, will have a rhombohedral unit cell. If the material is deposited with an (111) orientation, the electromechanical coefficient, d33, will be minimized.